The following U.S. Patents and publication in scientific literature are of relevance to the subject matter of the present invention:
3,922,485-11/1975 Starkweather et al. 178/7.6 PA1 4,040,096-8/1977 Starkweather 358/302 PA1 4,405,733-9/1983 Williams et al. 430/345
D. Chemla, J. L. Oudar and J. Zyss, L'Echo des Rescherches (Intern. issue) 47, (1981).
D. J. Williams, Ed. Non linear Optical Properties of Organic and Polymeric Materials, ACS Publish, (1983).
A. Dulcic and C. Flytzanis. Opt. Com. 25, 402 (1978).
G. R. Meredith, V. A. Krongauz and D. J. Williams. Chem. Phys. Lett., 87, 289 (1982).
F. P. Shvartsman and V. A. Krongauz. Nature, 309, 608-611 (1984).
H. Kelker and R. Hatz. Handbook of Liquid Crystals, Chemie, Weinheim, (1980).
Gale, D. J. Wilshire, J. F. K., J. Soc. Dyers Colour 90, 97 (1974).
Tarbell, D. S. Yamomoto, Y., Pope, B. M., Procl. Natl. Acad, Sci., USA 69, 730 (1972).
Itoh, M., Hagiwara, D., Kamiya, T., Bull Chem. Soc. Jpn. 50, 718, (1977).
Hinnen, A., Audic, C., Gautron, R., Bull. Soc. Chim. France, 5, 2066, (1968).
Lundt, B. F., Johansen, N. L., Volund, A., Makussen, J., Int. J. Pept. Protein Res., 12, 258, (1978).
Hassner, A., Alexanian, V., Tetrahedron Letters, 46, 4475, (1978).
Orahovats, A. S., Radeva, T. Zh., Spassov, S. L., Comp. Rend. Acad. Bulg. Sci., 26(5), 663, (1973).
When light is propogated through an optical dense medium, the induced electronic polarization of the material in an electric field E can be represented by the following expression: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3 + EQU P=P.sub.1 +P.sub.2 +P.sub.3 +
where the .chi.'s are coefficients of successive powers of the field, and .chi..sup.(n) is usually about 10.sup.5 .chi..sup.(n+1). This means that the high order terms will be important only at very high fields. P.sub.1 is the linear polarization and P.sub.2 is the first non-linear polarization. Symmetry considerations require that P.sub.2 =0) if the material has a center of symmetry associated with it. Finite values of P.sub.2 gives rise to new phenomena with considerable practical importance in laser-related technologies, optical communications and information processing (D. Chemla, J. H. Oudar and J. Zyss, L'Echo des Recherches (Intern. issue) 47, (1981). Materials with high second-order non-linear properties (.chi..sup.(2) &gt;&gt;0) are generally based on molecules having high first hyperpolarizabilities. Originally only inorganic crystals were used in optical devices based on these properties. However, recent research has led to the discovery of some organic crystals which have second-order non-linear optical coefficients several orders of magnitude larger than those of the conventional inorganic ones, and with much larger damage thresholds (D. J. Williams, ed., Nonlinear Optical Properties of Organic and Polymeric Materials, ACS Publish., 1983).
One example is 2-methyl-4-nitroaniline crystals with .chi..sup.(2) =1.25.times.10.sup.-6 esu. However, it is difficult to grow single crystals of this material of good quality, and not feasible to prepare thin films.
Recently it was shown that the molecular hyperpolarizability of some merocyanine dyes is surprisingly high (A. Dulcic and C. Flytzanis, Opt. Com. 25, 402 1978). If one uses this value and assumes that the merocyanine molecules are aligned in the bulk phase such that the material is noncentrosymmetric and that all of the molecular dipoles point in the same direction, .chi..sup.(2) would be approximately 10.sup.-5 esu. This would be the largest known value for nonlinear optical material and would enable the use of these materials in a variety of practical devices.
One of the most useful effects for investigation of optical non-linearity of material is second harmonic generation, SHG, which manifests itself as the conversion of light frequency 2.epsilon. on propagation through a suitable material.
There is known an electro-optical modulator, containing usually Pockel's cell which can be used in scanning devices. Such a device can transfer video information to a scanned medium by a scanning system, see for example, U.S. Pat. Nos. 3,922,485 and 4,040,096. The materials which may be used in such electro-optical modulators include crystals of KHPO.sub.4, LiTaO.sub.3, LiNbO.sub.3, BSN, etc. The property of these materials which make them useful in a device such as the Pockel's cell is their first non-linear polarization.